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Thread: Max fuel economy?

  1. #41
    convert2diesel Guest

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    Dieseldummy:

    Yes you are right. The higher volumes and pressures of the 4 piston pump do assure a better spray pattern but at what cost? The DB2 already sucks up close to 3 HP with 2 pistons. What the extra pistons would add I can only guess.

    The point I'm trying to make is that the boys at GM did a pretty good job of sizing the pump to the engine in order to do the job they intended. Stanadyne states that the DB2 is available in sizes up to 25HP per cylinder (4911???). Though I doubt the head design is capable of 200 HP, it does look like Dr. Lee has come real close without a turbo. This means that the engine, if made properly can flow enough air to accept this amount of fuel without an added push.

    Now back to the topic. Assuming your trucks require 25 to 30 HP to maintain 60MPH (just guessing at the numbers here) then using the formula of .43 lbs. of fuel per shaft HP per hour, then this engine will require 12.9 lbs of fuel to travel 60 miles. Based on 6.5 lbs/gallon, that gives us 1.98 gal per hour or about 30 miles to the gallon (sound familiar?)

    That also means the pump (regardless of chamber size or the number of pistons) is only pumping out a fraction of it's capabilities to maintain this power level. If the pump and injectors are sized and matched I can't see a significant gain in bumping up the fuel delivery.

    Just my thoughts

    Bill

  2. #42
    convert2diesel Guest

    Talking

    Morepower:

    Can we add family station wagons to this scenario??

    Bill

  3. #43
    dieseldummy Guest

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    MP, as to scenerio 3 I have to say it's all about the middle ground. 3:73 gears, stock tire size, with a 4L80E tranny or 4:10 gears, bigger tires size, also with the 4L80E. Would prefer the first. If 4x4 is a must then have it, but if you can get away with sandbags and 2wd in the winter do it. Run the '97 cooling mods with 195 thermostats in both holes. When not towing run conservative tire pressures for ride quality, but not low enough to create extra rolling resistance. When towing run them up to the max in the rear and a little higher in the front. Engine wise I say it's all about the injection system. Run a performance chip, or modified IP, but drive it like it only has the factory 180 horse power. Run high pop injectors, I have found that going from stock 6.5 injectors to stock 6.2 injectors has lost me 2-3 mpg. I will also have to add here that it has been my personal experience that the turbo has a big part in the economy equation. If financially feasible run a different turbo, or modify the stock GM series wastegate to open under low throtle conditions. Example: crusing down the road at 65 mph doesn't require 6 lbs of boost...The less restricion in the exhaust sytem the better. On one final note, I beleive that the quicker engine, transmission, T-case, diferentials, ect. come up to temp and stay within 5-10 degrees the better the efficiency of the vehicle will be.

  4. #44
    dieseldummy Guest

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    convert2diesel,
    I agree with you whole heartedly, I was just making an attemp to think outside the box. The only reason I suggested the DB-4 was for maximum sustainable pop pressure. What effect the 2 added plungers would have I don't know either. You are most likely right in that there isn't a whole lot to be gained from the motor, and focus should be on the rest of the drivetrain and aerodynamics.

  5. #45
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    I can usually find tech info that can either support or weaken a particular point of view.

    I did a little digging today looking for information regarding whether a turbocharged or a naturally aspirated diesel engine delivered better fuel economy in light-load applications, such as our scenario #1. The following information is generic in nature, not specifically targeting the 6.2/6.5 diesel engines. However, this might be useful to help us learn more about the subject. Seems Heinz Heisler isn’t all that secure in one particular point of view either.

    Excerpts from Advanced Engine Technology by Heinz Heisler

    Argument for naturally aspirated:

    Page 316:
    “Consequently, under light load and low engine speed conditions the energy released along with the exhaust gases will be relatively small and is therefore insufficient to drive the turbine assembly at very high speeds. Correspondingly, there will be very little extra boost pressure to make any marked improvement to the engine’s torque and power output in the low-speed range of the engine. Thus, in effect, the turbocharged engine will operate with almost no boost pressure and with a reduced compression ratio compared with the equivalent naturally aspirated engine. Hence, in the very low speed range, the turbocharged engine may have torque, power outputs and fuel consumption values which are inferior to the unsupercharged engine.”

    Argument for turbocharging:

    Page 343:
    “Turbocharging diesel engines can reduce the specific fuel consumption from about 3% to 14% in the engine’s speed range. The reduction in fuel consumption becomes more marked as the engine’s load is reduced, as can be seen in the family of constant load (b.m.e.p.) curves ranging from

  6. #46
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    MP:
    THANK YOU for the fuel curve, it was one of the first things that I wanted to see when joining this board.
    http://forum.thedieselpage.com/cgi-b...=002952#000000

    If anyone is interested in the cummins 4bt fuel curve (the motor in gm bread trucks) I can post it. But I can tell you that at redline, it's only .40, and @ max torque, it's .37 --- Much better than our 6.2's. So the best method of getting better mileage is to use a gm / cummins 4BT. (grin)

    I also started a thread on this on the 6.5 side.
    http://forum.thedieselpage.com/cgi-b...=006710#000024

    From my investigating, it appears that larger, heavier duty transmissions absorb more HP, leading to lower fuel economy. Also RP ratio, the coating (if any) on the RP as well as the oil used contributes to loss of HP / MPG.

    My opinion on max mileage (assuming you HAVE to have an auto tranny)
    Stock J code 6.2 NA, 4" fresh air intake, dual exhaust, 700r4, 4.1 gears, 265-85r16, mobile1 in the rear end.

    The fuel curve seems to disprove the 1800 conventional wisdom for max fuel economy, in fact the sweet spot appears to be about 2200 -- any thoughts on this???

    One big unknown for me, and I've posted on this as well, http://forum.thedieselpage.com/cgi-b...=002850#000004
    is a dual exhaust on a NA 6.2 in terms of fuel efficiency. It is common knowledge that a properly tuned balance pipe, creates a vacuum on one bank from the exhaust pulse on the other bank. This in effect creates a turbo effect (Think 2 cycle motorcycle effect), without wasting the energy spooling a turbo. The effect is very rpm dependant, but if it were tuned to the cruising rpm, it would dramatically increase volumetric efficiency, and in theory MPG. Thoughts?

  7. #47
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    I know that my 83 NA 6.2 conversion won't get out of it's own way. Yet it gets 25mpg. Everyone I heard, that has put a turbo on a 6.2 confess they can't help pressing harder on the acc. pedal. I know that kills mileage. I own 3 VW TDIs they all have intercoolers and turbos why? Did VW have alot of parts laying around? I've been told you can't run a diesel engine lean. So why not cram lots of cool air into the engine and a little fuel? I knew a guy who had a banks on a 85 c1500 pickup pulled a motorcycle trailer across the country, got 28mpg. I may not like the mpg of my van with a turbo but I'm going to try it. I had driving it in traffic!
    95 excab K2500 354,000+miles
    />89 excab K1500 489,000<br />84 conv. van 225,000<br />97 Passat TDI 304,000<br />96 Passat TDI 178,000<br />98 Jetta TDI 165,000<br />All diesel

  8. #48
    convert2diesel Guest

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    Along the lines of gearing has anyone had any experience in building up a THM350C? When researching the powertrain for the Buick, with the goal of keeping the engine in the 1,500 to 2,100 RPM range, I was tempted to go with the 400 coupled to a dual range, add-on overdrive. This would give me the advantage of an overdrive plus the ability to gear split all forward gears when towing the trailer.

    While 6 forward gears was enticing, the thought of heat build-up and efficiency loss through a non-lockup convertor made me back-off that idea. While researching build-ups for the 700R4 I found that many of these places also did 350s for the throttle jumpers. Maybe if you could build one of these strong enough and couple it to a low stall lock-up convertor and then run it through the overdrive unit, the same results could be realized.

    Is this viable?

    Bill

  9. #49
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    RE: Cylinder Heads

    Enjoying this thread, as I have a project Blazer in the planning/parts gathering phase, with this theme. I would be interested in a follow-up discussion on what is the best cylinder head build-up for 6.2 NA engine. New to diesel/6.2.
    (A little bit of experience w/ gas engines)
    Why was the "6.2L "C" series cylinder heads" recommended above? Assume this is any "C" engine head? Why not "J" series heads? Assume their are many areas we could discuss,e.g. best valves, valve preparation, seat angles, throat cut angles/depth, head porting tips ( critical areas to work/ areas not to touch), valve lift vs power development ( what lift range effects power most). I am ready to turn on the flow bench and get the grinder cutting!
    thanks,
    Les
    03\' 3500 GMC Crew Cab Duramax<br />86\' one ton SRW 6.2<br />85\' Blazer 6.2 under development

  10. #50
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    "C" series cylinder heads were designed at a time when fuel economy was a huge factor in marketing GM full-size pickups. Mostly, this involves precups with the smallest ports. 6.5TD heads could be rebuilt using "C" cups. Dr. Lee's Avant 6.5 comes very close to the ideal fuel economy engine.

    Saw a discussion about the NV3550 the other day on TV. This is an aluminum cased 5-speed, rated for 450 lb-ft of torque. This might be a good way to go for the gear-bangers out there, to use for a fuel economy rig.

    Automatics with a locking TC come close to the efficiency of an OD manual, but the manual is best.

    MP

  11. #51
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    Here's the typical gm bread truck cpl.

  12. #52
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    some math, assuming a typical suburban, that gets 20mpg at 60mph, and gearing that has the engine running somewhere between 1800-2400.

    At 60mph getting 20mpg, that's 3 gallons in an hour * 7.1lbs/gal for #2 fuel = 21.3lbs of fuel / hr.
    .41lbs/hp*hr / 21.3lbs = 60hp*hr used for a gm6.2
    .37lbs/hp*hr / 19.22lbs = 60hp*hr used for a b3.9
    cummins b3.9 @60mph gets 22.2mpg, that's 2.7gallons in an hour * 7.1lbs/gal for #2 fuel = 19.22 lbs.

    So, what't that tell us? Even a direct injection cummins, only gets us 2.22 mpg bump, assuming the same load. The big thing to look at, is why does it take 60hp to move a typical suburban down the road? If you are looking for max economy, looking at improvements in the engine is nickle and dime stuff (with the exception of using an electric vs. mechanical fan), looking at reducing the power requirements (reducing driveline losses, rolling resistance losses, and aerodynamic losses) gets you much further down the road.

    Anyone done any airdam testing on mpg on a typical suburban?

  13. #53
    moondoggie Guest

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    Good Day!

    convert2diesel: I thought Jet offered a 700R4 warranted to 440 lb/ft, which would more than cover this application. This would provide the lockup TC, which would seem better for mpg than any non-lockup TC. I've also heard it said that the 700R4 has lower 1st & 2nd gear ratios than the 400 or 4L80-E, which, while not being as elegant as gear splitting, would help with towing and/or hill climbing. The looser TC allowed by having lockup would help here too.

    I really know nothing about trannies, just parroting what I've heard elsewhere. I know you folks will fix my errors.

    Blessings!

    Brian Johnson, #5044

  14. #54
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    http://www.airtab.com/

    This looks like the best bet for cleaning up the aerodynamics of a suburban. Hey it works for tractor trailers, why not a sub?

  15. #55
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    MP thanks for the reply on the "C", at least one more question, what time period was this? mid to late 90's.
    Jbell, the fuel, torque, HP, vs RPM curves are outstanding!! Loads of good information.

    About lowering HP needs, the 10 bolt rearend has the lowest HP lose, better then 12 bolt, 14 bolt is not even close. For the 10 bolt rear you can use aluminum drums from Trans Ams, Formula, certain Buick Gran Nationals, and a limited number of station wagons that came w/ this rear. Lowering the rotating weight of drive train helps mileage, and more then other weight in the vehicle. I think it may be something like 1 pound from drivetrain rotating mass is equal to 4 pounds of static vehicle weight. Therefor tires, wheels, drive shaft, transmission and differential rotating parts are all potential areas for improving milage by reducing their weight.

    Another area is friction in the power train.

    Tire design and size are also areas of measurable improvemnent as mentioned above.

    For me the biggest improvement would be to get rid of the hills here in Kentucky!

    thanks for the information,
    Les
    03\' 3500 GMC Crew Cab Duramax<br />86\' one ton SRW 6.2<br />85\' Blazer 6.2 under development

  16. #56
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    The 6.2L "C" series diesel engine was produced from 1982 through 1993. The recent listing of the "Fuel Miser" 6.5 engine by GM Powertrain is the most current rendition in design configuration(though lacking some emissions control devices).

    Regarding scenario #3, needing to tow occasionally means the engine needs a turbo. If only light trailers and for infrequent towing duties, 3.73 gears, "C" cups, and a DB2-4911/6.5 injectors. If towing more or more often, 4.10 gears and possibly TD cups. Based on lots of discussions with lots of owners, there appears to be about a 2-mpg difference in cups alone.

    If towing more than 5-7K, and doing it often, I'd then go to an 18:1 engine and all the other stuff necessary to keep things cool and make More Power, which is now beyond the scope of this thread.

    MP

  17. #57
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    if you are going for max fuel milage, with the least possible drag, who are you going to have align the truck.....Sears? I would suggest NOT letting anyone with a computer alignment machine touch the vehicle in question. Those things are the devil and assume the rear end is square to the centerline of the lower front susupension pickups. But this suggestion is only if you decide to get serious about this deal.

  18. #58
    CareyWeber Guest

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    Originally posted by grape:
    if you are going for max fuel milage, with the least possible drag, who are you going to have align the truck.....Sears? I would suggest NOT letting anyone with a computer alignment machine touch the vehicle in question. Those things are the devil and assume the rear end is square to the centerline of the lower front susupension pickups. But this suggestion is only if you decide to get serious about this deal.
    This seems like a worth while thing to do for best MPG.

    Someone said once here that mirrors are a big factory in fuel mileage.

    I think tire pressure is big deal.

    Carey

  19. #59
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    MP,
    Thanks for the additional information on "C" cylinder heads, are these heads the same or different from the "J" engines of those years.

    Have five CUCV Blazers w/engines which I will us to build my project Blazer. They all have low mileage 25 to 45K miles and the model years are 83' and 84'.
    thanks,
    Les
    03\' 3500 GMC Crew Cab Duramax<br />86\' one ton SRW 6.2<br />85\' Blazer 6.2 under development

  20. #60
    dieseldummy Guest

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    The heads are the same, the only difference is the precups, which are interchangable...

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